When reading sunscreen labels, pay attention to the sun-protection factor, typically listed as SPF plus a number. A product with an SPF 15 (the minimum recommended by the Skin Cancer Foundation) will block about 93 percent of UVB rays—the ones that cause tanning and burning. SPF 30 will block about 97 percent, and even lotions with an SPF of 50 or higher probably block no more than 99 percent, according to the Skin Cancer Foundation. "Higher doesn't necessarily mean better," says Joel L. Cohen, MD, chair of patient education for the American Society of Dermatologic Surgery. "Because to get that higher SPF number, the product may contain a higher concentration of potentially irritating chemical ingredients."

The other caveat? SPF refers only to the amount of protection you're getting from UVB rays. While ample proof exists that UVA rays do just as much damage to skin—causing signs of premature aging as well as raising the risk of skin cancer—knowing exactly how much protection you're getting from UVA rays remains elusive. Hedge your bets by looking for a sunscreen that promises "broad spectrum" protection, which means it contains ingredients that filter out both UVA and UVB rays.

How you apply sunscreen is just as important as what you apply. According to the recommendations of the Skin Cancer Foundation, you need to apply a full ounce of sunscreen to protect your entire body adequately, and you need to put it on about 30 minutes before heading outdoors so the ingredients can bind with your skin. For a day outdoors, reapply about every two hours—more often if you take frequent dips or sweat profusely.

And when applying sunscreen, don't neglect your scalp. A non-oily spray sunscreen is perfect for spritzing on your part. "A visor is great for keeping sun off the face, but since you can get skin cancer on your scalp, it's better to wear a brimmed hat to cover your scalp and head," says Cohen. Hair—unlike the scalp—can't get skin cancer, but dermatologists warn that hair can dry out and lose its luster when exposed to too much UV light.